Nanoparticles are very small particles typically ranging in size from as small as one nanometer to as large as several hundred nanometers in diameter. Their small size allows nanoparticles to be exploited to produce a variety of products such as dyes and pigments; aesthetic or functional coatings; tools for biological discovery, medical imaging, and therapeutics; magnetic-recording media; quantum dots; and even uniform and nanosize semiconductors.
Nanoparticles can be simple aggregations of molecules or they can be structured into two or more layers of different substances. For example, simple nanoparticles consisting of magnetite or maghemite can be used in magnetic applications (e.g., MRI contrast agents, cell separation tools, or data storage). See, e.g., Scientific and Clinical Applications of Magnetic Microspheres, U.Häfeli, W. Schütt, J. Teller, and M.Zborowski (eds.) Plenum Press, New York, 1997; Sjøgren et al., Magn.Reson. Med. 31: 268, 1994; and Tiefenauer et al., Bioconjugate Chem. 4:347, 1993. More complex nanoparticles can consist of a core made of one substance and a shell made of another.
Many different type of small particles (nanoparticles or micron-sized particles) are commercially available from several different manufacturers including: Bangs Laboratories (Fishers, Ind.); Promega (Madison, Wis.); Dynal Inc.(Lake Success, N.Y.); Advanced Magnetics Inc.(Surrey, U.K.); CPG Inc.(Lincoln Park, N.J.); Cortex Biochem (San Leandro, Calif.); European Institute of Science (Lund, Sweden); Ferrofluidics Corp. (Nashua, N.H.); FeRx Inc.; (San Diego, Calif.); Immunicon Corp.; (Huntingdon Valley, Pa.); Magnetically Delivered Therapeutics Inc. (San Diego, Calif.); Miltenyi Biotec GmbH (USA); Microcaps GmbH (Rostock, Germany); PolyMicrospheres Inc. (Indianapolis, Ind.); Scigen Ltd.(Kent, U.K.); Seradyn Inc.; (Indianapolis, Ind.); and Spherotech Inc. (Libertyville, Ill.). Most of these particles are made using conventional techniques, such as grinding and milling, emulsion polymerization, block copolymerization, and microemulsion.
Methods of making silica nanoparticles have also been reported. The processes involve crystallite core aggregation (Philipse et al., Langmuir, 10:92, 1994); fortification of superparamagnetic polymer nanoparticles with intercalated silica (Gruttner, C and J Teller, Journal of Magnetism and Magnetic Materials, 194:8, 1999); and microwave-mediated self-assembly (Correa-Duarte et al., Langmuir, 14:6430, 1998). Unfortunately, these techniques have not proven to be particularly efficient for consistently fabricating nanoparticles with a particular size, shape and size distribution.